kabroui

I remember taking off from LPB (over 13000') a few years ago when AA flew MIA-LPB- VVI-MIA. I think they do some special adjustments to the engines due to the elevation in order to take off in LPB (which they reversed in VVI.) Seemed like it took forever to get off the ground and after that the ascent was very slow.. meanwhile the ground drops off around 1000 feet at the end of the runway. Weight restrictions as well and some passenger moves to balance the weight. Interesting place to depart from.

jsloan

The FAA regularly updates these numbers based upon samples of people's weights and the weights of their carry-ons. Getting an exact weight normally isn't necessary, but being in the correct ballpark is, and it's a responsibility that is taken seriously.

Jeff767

As mentioned above the weights are usually accurate. You know this from a pilot standpoint by your fuel burn. If you under burn you are light and if you overturn you are heavy. The fuel burns are remarkably accurate.
As far as performance density altitude is critical. RNO with a 97 degree temp and standard pressure has a density altitude of 8000 feet. A jet engine will lose 25% or more in available thrust at 8000 DA. Combine that with higher actual speeds required for the same amount of lift and you have a huge loss in takeoff performance.

JimInOhio

Does DEN have many takeoff weight restrictions? I'm sure it may become an issue on a super hot day but they have one of the longest runways in the world at 16,000 feet. All of the other runways are still quite long at 12,000 feet.

tods27

The long runways were specifically put in to help with heat/altitude issues. There also isn't a steep climb needed after takeoff from DEN, which helps. Even for routes to the west, they can take off to the North or South, then turn West (I can't speak to specific routes), depending on winds and other factors. I don't know how often there are restrictions at DEN, but I would assume that most 90+ days would require some percentage of planes to have restrictions. Those are fairly common in the Summer here in Denver.

8000 feet made me think of Vail, which made me thing of flying in/out of EGE, where you almost always had 757s or Dash 8s for their higher power/weight ratio.

mduell

The wing also loses 25% of the lift, so you need ~12% higher takeoff speeds. Less thrust getting to higher speeds eats up a lot of runway.

Some aircraft will have takeoff weight restrictions regardless of runway length since they can run into tire speed limits.

For example from the 737 NG ACAP, the 737-900ER can't use more than 13-14k ft of runway when hot and high due to tire speed limits:

Jeff767

Denver does not have obstacles in the climb path which helps. The 16,000 foot runway certainly helps but there are sometimes other considerations on a hot day like tire speeds. The high density altitude leads to very high ground speeds before you can rotate. This can require a weight restriction to avoid exceeding max tire speeds.

LarryJ

Before each flight, calculations are performed to find the max allowable takeoff weight for the flight. There are several limitations which are considered and the one which is the most restrictive for current conditions becomes controlling for that flight.

As has been discussed, it can be takeoff performance, landing weight, or zero-fuel weight. Other possible limitations are enroute terrain clearance, structural takeoff weight, and fuel-limited weight.

Structural takeoff weight is the simplest. That is the highest weight with which the airplane is certified to takeoff. On the 737s that I fly, that ranges from 154,500, for the 737-700, to 194,700, for the Max 9. To make the numbers easier to work with, we divide by 1,000 and call those 154.4 and 194.7.

One of the more common restrictions is landing weight. The max certified landing weight on the 737s range from 129.2 to 163.9. If you're flying a short flight in a 737-700, with a planned fuel burn of 5.0 (5,000lbs of fuel), your landing weight limit would be 129.2 + 5.0 = 134.2 which is far below the structural limit of 154.4. That flight would be landing weight limited.

Fuel-limited flights don't happen very often, but would be a case with a long flight where you reach the point where the fuel tanks are full. Adding more passengers or cargo would increase the fuel burn but the tanks are already full.

Enroute performance limits apply when your flight is crossing mountainous terrain. If you lose an engine in cruise, you need to be able to maintain an altitude above the highest terrain along your route to ensure that you can safely reach an airport. We see this limit on most west coast to Midwest/east coast legs, though it may not be the most restrictive limit.

Zero fuel weight (ZFW) is a certification limit. All weight over the ZFW must be in fuel. This limit is based on structural considerations as weight in the fuselage is carried by the wing attachment structures and weight in the wings (i.e. fuel) is not. The ZFW is the limit to the weight that can be carried in the fuselage and ranged from 121.7 to 156.5 on the 737 fleet. If we were flying a Max 9 on a flight with a 15.0 fuel at takeoff, our ZFW limit would be 156.5 + 15.0 = 171.5. When we have a ZFW limit we can sometimes reduce our fuel, if sufficient reserves remain, to increase our payload. If the airplane has already been fueled, it might be possible to burn some extra fuel during taxi or fly at a lower altitude to increase the enroute burn. There's a limit to this, though, as you still need to leave adequate reserve fuel at landing.

Takeoff performance is the most complex limit and includes all of the factors below.

Tire speed. It's not usual to have tire speeds approaching 190 knots (219 mph) at liftoff from Denver. The tire speed limits on the 737 is 195 knots, for the NG, and 204 knots, for the Max. Carrying more weight will increase the speed at liftoff. When you reach the point that more weight would result in exceeding the tire speed limit you've reach your tire speed takeoff weight limit.

Brake energy limit is similar to tire speed but is based on the amount of energy the brakes would have to dissipate in a rejected takeoff at takeoff-decision speed (V1). The heavier you are, and the faster you are going at V1, the more energy there is to dissipate and the hotter, and thinner, the air, the lower the ability of the brakes to dissipate the heat.

Accelerate-stop distance or runway limit. For each takeoff, you must be able to accelerate, have an engine failure, initiate the reject by V1, and stop safely on the remaining runway. The heavier you are, the longer that distance will be and that distance must be no more than the available runway length. This is why Denver has that one 16,000' runway.

Accelerate-go distance, or climb limit. For each takeoff, you must be able to accelerate, have an engine failure at V1, then continue to takeoff safely, clearing any obstacles. As weight increases, your climb angle and climb rate will decrease. Mountainous terrain will restrict your climb limit weight when it requires a higher than standard climb gradient.

That covers (most of?) the limits. Here's what we do about them.

One of the first things I look at on a flight release is the planned takeoff weight as compared to the projected takeoff weight limit. If there's a large margin between them we don't have to worry about a weight restriction. If they are close, I need to look more closely at the factors driving the specific restriction and the fuel planning to ensure that payload can be maximized for the conditions.

A landing weight limit is all about fuel remaining so a close look at the weather and possible enroute delays is made to ensure that we are carrying all of the fuel that we need, but not excess fuel.

With an enroute terrain limit we can often run a new flight plan using "method 2" which is a more detailed analysis of the terrain for the specific route where "method 1" has a less specific analysis and just "rounds up", you could say. With method 2 we will have specific alternates along the route with defined changeover points where we'll switch to the next alternate.

There are a number of ways to mitigate takeoff performance limits.

The first, which is automatic, is to increase takeoff thrust. Almost all takeoffs are at reduced thrust, to increase engine life. The performance software automatically increases takeoff thrust based on weight and conditions. Once that limit is reached, we can do a "bleeds off" takeoff for which the engine bleed air is shutoff for the takeoff which allows the engines to produce more thrust. For such a takeoff, we leave the APU running to use the APU bleed air to run the airconditioning and pressurization for takeoff. After takeoff, we reconfigure to engines bleeds and shutoff the APU. This is used with some regularity at Denver during the summer.

Next is to address the runway limit and climb limit. We can use a longer runway, if available, to increase the runway limit or choose a runway with headwinds instead of tailwinds. We can also increase the takeoff flap setting as that lets us liftoff with a shorter ground run. The 737 can takeoff with flaps at 1, 5, 15, or 25 degrees. When it is a climb-limit issue we can use less flaps. Flaps will shorten the ground roll but they decrease the initial climb rate so more flaps increases your runway limit while reducing your climb limit.

For performance-based limits, our max weight changes with temperature, pressure, and wind so we don't get a final weight limit number until closer to takeoff.

I had a potentially weight restricted departure from Denver last week. Scheduled departure was just after 8am so the temperature was still increasing and a new weather observation would be released before takeoff. Load planning had alerted dispatch and customer service to a possible weight restriction and CS was soliciting volunteers in case we couldn't get everyone onboard. I guessed that the temperature on the next observation would increase 3°C before takeoff. I calculated our best weight using the most favorable runway/winds and with bleeds off. We sent that number to dispatch who updated our release with the new max weight that I had calculated. That number then went to the load planner who adjusted the weight restrictions (max pax and bag count) given to customer service.

When the new temperature came out it had increased exactly 3° (I got lucky on my guess). By that time, we were just closing doors and preparing for pushback. We were able to get all customers, and all of the non-revs, onboard. If the temp had increased more than 3° I would have had to revise the numbers and some of the non-revs might have had to be off loaded. The final weight was a bit under planned so we were able to do a bleeds on takeoff from the preferred (ATC) runway for our departure instead of the runway with the highest performance limit.

WheelsUpGal

I used to fly out of RNO on Continental as a non-rev. I think Continental only flew to two places out of Reno but I always flew to Denver to get anywhere else. And both RNO and DEN could be a hassle with the weight restrictions in the summer months. I loved driving down from Tahoe early in the morning to catch (maybe) an 8am flight. Only missed the cut one time in 3 years. Missed an old boyfriends wedding. No big loss! Love the in depth explanations and chart!

bmwe92fan

As someone that is old enough to remember when Tahoe had commercial jet service -- that was one of the most stressful landing places as a passenger I've ever had. We had many sales meetings there -- take off was fine, landing on such a short runway was always interesting in an md-80.... I always wondered if a go-around was even possible given the terrain since you always landed towards the mountain... That drive from Reno to Tahoe as a passenger was always motion sickness inducing -- unless I was driving....

mahasamatman

If you have the chance, have an experienced pilot take you on the "canyon approach" to Shelter Cove on the California coast. My instructor told me that if I did it right, it would scare him, and it did.

Though my favourite commercial landing was on Saba. It has the shortest commercial runway in the world (1,312 feet), and the pilots generally land and get the plane stopped in the first third.

bmwe92fan

Propeller aircraft have an amazing ability to stop on a dime -- for years when I was young calling on Walmart I would take the B1900 into Fayetteville airport which was in a valley between the hills and did not have ILS. We would land and stop so quick it always amazed me -- but what terrified me was how much the plane was moving sideways even right up until landing....

My absolute worst "landing" experience ever was in Denver -- I was listening on Channel 9 and the pilot missed three landings in a row and lost confidence -- the co-pilot took over and landed the plane -- the reason it was so scary to me is that I was listening to the conversation with ATC -- the pilot was insisting there was wind shear and ATC was saying that every plane, every time, before and after had zero issues -- it was only our plane. I was quite happy to get off that flight and seriously considered not listening to channel 9 any more - I could literally hear the fear in their voice and it scared the you know what out of me...